Development And Evaluation Of Magnetic Resonance Contrast Agents

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Abstract

Various MRI contrast agents have been developed for cellular MR imaging over the past few years. In general most contrast agents used for cell labeling are superparamagnetic iron oxide nanoparticles (SPIO) that result in T2 and T2 shortening of the surrounding tissues and hypointense regions on T2 weighted MRI. Paramagnetic contrast agents based on gadolinium can be used to label cells. Gadoliniumfullerenol (Gd@C82) is a paramagnetic agent that has unique NRM relaxation time properties was evaluated as a possible intracellular contrast agent to label cells. Protamine sulfate transfection increased cell uptake of Gd@C82 fullerenols. The label was distributed in endosomes in the cytoplasm as shown by electron microscopy. High viability was shown for all cell lines and normal differentiation capacity was shown for mesenchymal stem cells (MSC) although there was some effect of Gd@C82 fullerenol on cell proliferation. Injections of labeled cells into muscle could be detected as hyperintense region compared to surrounding tissue on MRI. However, Gd@C82 altered the viability and proliferation of mesenchymal stem cells and therefore further investigation of Gd fullerenols for tracking cells needs to be further investigated to determine if it is a useful for cellular imaging. Viral capsids and recombinant protein cages have the potential for combined celltissue targeting, drug delivery and imaging. Two approaches were developed to fuse high affinity Gd3 chelating moieties to the surface of the Cowpea chlorotic mottle virus (CCMV) capsid. In the first approach, a metal binding peptide has been genetically engineered into the subunit of CCMV. In a second approach GdDOTA was attached to CCMV by reactions with endogenous lysine residues on the surface of the viral capsid. T1 and T2 ionic relaxivity rates for the genetic fusion particle 50100x greater than GdDOTA. The combination of high relaxivity, stable Gd3 binding and large Gd3 payloads indicates the potential of protein cages as high performance contrast agents and the possibility to modify these agents to increase binding to target cells following intravenous injection. Further research using recombinant selfassembly proteins with iron or gadolinium cores may be useful as contrast agents for cellular imaging. Multifunctional fluorescent and SPIO nanoparticle contrast agents provides the ability to track labeled cells with cellular MRI and fluorescent microscopy. Several groups have reported functionalization of dextrancoated SPION but not with a simple approach. We have developed a simple oxidation approach was used to introduce functional groups onto dextran coated SPION allowing for the reaction of the nanoparticles with several different fluorescent dyes. The fluorescent SPION were stable and successfully used for cell labeling. HeLa cells and mesenchymal stem cells were labeled with fluorescent dextrancoated SPION such as fluorescent ferumoxidesprotamine sulfate complexes (FLUO FEPro) and quantitative cellular iron incorporation was similar to what was reported without fluorescent tag. FLUO FEPro labeled cells showed stronger fluorescent images confirming successful fluorescent dye conjugation onto dextrancoated SPION.